Increasing integrity of watermarks using robust features

ABSTRACT

This invention relates to a method of embedding a watermark pattern with a payload in a time dependent information signal, comprising the steps of determining a number of robust signatures in the information signal, creating the payload being dependent of at least one of the number of robust signatures and a predefined message, embedding the watermark pattern according to the payload in the information signal. The invention also relates to a corresponding method and arrangement for detecting a watermark pattern in an information signal.

The invention relates to a method and an arrangement for embeddingauxiliary data, such as a watermark in an information signal, e.g. avideo signal, an audio signal, or more generally, multimedia content.The invention also relates to a method and arrangement for detectingsaid watermark pattern and a device for recording and/or playing back aninformation signal.

The auxiliary data may e.g. be a digital watermark, which preferably(but not necessarily) is an imperceptible label that is embedded/addedto an information/host signal e.g. comprising multimedia content, video,audio, etc. The label may contain for instance copyright information,the name of the owner of the material, rights for a user etc. Theinformation that may be stored in or derived on the basis of a watermarkis usually referred to as a payload and is expressed in bits. Note, thatsometimes the term ‘payload’ of embedded watermarks also refers to theamount of information that may be stored in or derived on the basis ofan embedded watermark. However, throughout the following the term‘payload’ refers to actual values (e.g. a bit-string) that aretransferred, embedded, derived, etc. on the basis of a watermark.

In most watermark schemes the watermark is a pseudo-random noisesequence (pn-sequence), which is added to a host signal/informationsignal in either the time, spatial or a transformed domain (e.g.Fourier, Discrete Cosine or Wavelet Domain). Watermark detection is thenusually based on a correlation between the watermark and the embeddedhost signal. In this case we have a 1 bit payload for the watermark,i.e. the noise sequence is either present or it is not present.

A recognized problem in the security of watermarking is the so-calledcopy attack. This attack estimates a watermark from an embedded hostsignal and subsequently the estimated watermark can be transplanted in asecond signal. If the second signal was originally unmarked then asignal is generated which in terms of watermarking assumes identity ofthe embedded host signal. Further if the second signal was alreadymarked with a watermark then the newly created signal might confusewatermark detectors.

In order to avoid the copy-attack one option is to make the watermarkdependent on the content of the host signal. This is done by extractinga robust signature (a set of robust features) from the content of thehost signal and make the watermark dependent on this signature, e.g. asdisclosed in patent application WO 01/39121. A robust signature is a setof variables that is representative of the essentials of the hostsignal. If e.g. the host signal is a video signal, then ideally a slightchange in the image represented by the video signal leads to no changein the signature, whereas a complete different image results in aradically different signature.

The above method leads to detection problems when the signature changesvery quickly such as a “flashy” video clip. In known watermarkingsystems, the watermark is embedded in frames of the host signal. Whendetecting the watermark in the host signal, all the frames in a timeperiod of e.g. 2 seconds are accumulated in a buffer. Thereby thewatermarks in each frame add up coherently, improving the signal tonoise ratio for the watermark detection. After the accumulation step thebuffer is correlated with the watermark pattern, and the result iscompared to a threshold. A problem with the above-described idea is whena watermark is detected in e.g. ten seconds of video material, thesignature might have changed four times in this period. In this case,the watermark pattern also changes four times in the detection period,and hence the watermarks do not add up coherently. This leads to alesser improvement in detection signal to noise ratio.

It is an object of the invention to provide a method and arrangement forembedding additional/auxiliary data in an information signal where themethod and arrangement solves the above-mentioned problems.

This is achieved by a method (and corresponding arrangement) ofembedding a watermark pattern with a payload in a time dependentinformation signal, comprising the steps of:

-   -   determining a number of robust signatures, each signature being        a set of robust features in the information signal, the robust        signatures being determined in a predetermined time interval of        said information signal,    -   creating the payload being dependent of at least one of said        number of robust signatures and a predefined message,    -   embedding the watermark pattern according to the payload in the        information signal.

The corresponding method (and corresponding arrangement) of detectingauxiliary data in an information signal comprises the steps of detectinga message in a time dependent information signal with an embeddedwatermark pattern, comprising the steps of:

-   -   detecting the watermark pattern in the information signal and        detecting the payload,    -   determining a number of robust signatures, each signature being        a set of robust features in the information signal, the robust        signatures being determined in a predetermined time interval of        the information signal,    -   decoding the message from the payload using at least one of the        number of robust signatures.

Preferred embodiments of the invention are defined in the sub claims.

Hereby, the information in the payload of the watermark depends on theinformation signal and the message can only be detected in combinationwith information from the information signal. Thereby it is not possibleto copy the watermark and use it on another information signal withdifferent information/content.

FIG. 1 shows an embodiment of an embedding arrangement according to thepresent invention;

FIG. 2 illustrates a schematic diagram of a detector according to thepresent invention;

FIG. 3 illustrates a detailed embodiment of a detector according to thepresent invention;

FIG. 4 shows a device for recording and/or playing back an informationmulti-media bit stream with an embedded watermark.

For the sake of convenience the invention will be described as a systemfor embedding/attaching labels, preferably invisible to the human eye,to video content but the teachings can obviously be applied to any othercontents including audio and multimedia. Additionally, an embodiment fordetecting labels is also described.

FIG. 1 shows an embodiment of an embedding arrangement according to thepresent invention. A source provides an information signal X to beembedded with auxiliary information/a watermark. The source may e.g.provide an image, an audio signal, a signal with multimedia content,etc. As an example used in the following, the information signal X istime dependent. Also shown is watermark embedding means (105), whichadds a watermark W to the information signal X.

The watermark W is generated by first extracting a number of robustsignatures from the information signal X in a predetermined timeinterval, each robust signature is a set of robust features extractedfrom the content of the information signal X. The extraction isperformed using signature extraction means (101) and the output of thesignature extraction means (101) is a signature S.

The signature S is used as input to the payload generating means (103)together with a message M. The payload generating means (103) determinea payload P as a signature dependent, invertible function f_(s)(•) ofthe message M.P=f _(s)(M)

The payload P and thereby watermark pattern W is embedded in theinformation signal P by the watermark embedding means (105) and aninformation signal (Y) is generated comprising the watermark pattern W.

In a specific embodiment the payload P is generated by concatenating themessage M and a function g(•) of the signature S, e.g.P=(M,g(S))=(P1,P2)

As mentioned above, a number of signatures are extracted from theinformation signal and examples of different methods of determiningwhich signature(s) to use during payload encoding will be described inthe following.

The signature could be chosen randomly, this would require memory meansin order to use the same signature when the payload is to be decodedduring watermark detection.

The signature could also be chosen between the number of signatures asthe n-th signature, e.g. the 5.th signature extracted from theinformation signal.

Alternatively, the signature could be chosen as the signature foundafter a predetermined number of seconds (in the interval of T secondsduring which the payload is kept constant).

As another alternative, it may be the most robust signature that ischosen, whereby the probability of failure during decoding woulddecrease. The most robust signature could be chosen using a predefinedrobustness measure.

Further the signature to be used could be chosen using a key-frame.

FIG. 2 illustrates a schematic diagram of a detector according to thepresent invention. Shown is an information/host signal Y, which as anexample is time dependent and possible contains a watermark with amessage M to be detected.

First a number of robust signatures are extracted from the informationsignal X in a predetermined time interval, each robust signature is aset of robust features extracted from the content of the informationsignal X. The extraction is performed using signature extraction means(201) and the output of the signature extraction means (201) is asignature S.

Further the payload P is extracted from the information signal Y using awatermark detection means (203).

The signature is used to generate an inverse signature dependentfunction f_(s) ⁻¹ and the message M is decoded from the payload P usingthe payload P as input to the inverse signature dependent function f_(s)⁻¹(•),M=f _(s) ⁻¹(P)

The decoding is performed by payload decoding means (205), which havethe signature S and the payload P as input and the message M as output.

The signature can be chosen according to the same methods as describedearlier in connection with the embedding process. When choosing thesignature it is important that the signature used during the embeddingprocess is also used during detection.

A more detailed embodiment of the watermark detector is shown in FIG. 3.The signatures are extracted from the information signal Y by thesignature extraction means (301). Then all the collected signatures {S1,. . . , Sn} from a predefined time interval are collected by signaturecollection means (302). Means for checking the integrity (303) thencombines the signatures {S1, . . . , Sn} with a payload extracted fromthe information signal using a watermark detector (304). Informationabout the integrity can then be used for controlling access restrictionsto the information signal.

The embedded information may identify, for example, the copyrightholder, a description of the content and/or rights associated with theuse of the content. In DVD copy protection it would allow material to belabelled as ‘copy once’, ‘never copy’, ‘copy no more’, etc. FIG. 4 showsa device, e.g. a DVD player, for recording and/or playing back an MPEGencoded bit stream with an embedded watermark. The bit stream isrecorded/stored on an information medium like a DVD disc 401. Therecorded signal is applied to an output terminal 403 via a switch 402.The output terminal 403 is connected to an external MPEG decoder anddisplay device (not shown). It is assumed that the DVD player may notplay back video signals with a predetermined embedded watermark, unlessother conditions are fulfilled which are not relevant to the presentinvention. For example, watermarked signals may only be played back ifthe medium 401 includes a so-called “wobble” key. In order to detect thewatermark, the DVD player comprises a watermark detector 404 asdescribed above. The detector receives the recorded signal and controlsthe switch 403 in response to whether or not the watermark is detectedand/or what the value of the auxiliary data/the message signifies.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.The word ‘comprising’ does not exclude the presence of other elements orsteps than those listed in a claim. The invention can be implemented bymeans of hardware comprising several distinct elements, and by means ofa suitably programmed computer. In a device claim enumerating severalmeans, several of these means can be embodied by one and the same itemof hardware. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage.

1. A method of embedding a watermark pattern (W) with a payload (P) in a time dependent information signal (X), comprising the steps of: determining a number of robust signatures (S), each signature (S) being a set of robust features in said information signal (X), said robust signatures being determined in a predetermined time interval of said information signal, creating the payload (P) being dependent of at least one of said number of robust signatures (S) and a predefined message (M), embedding said watermark pattern (W) according to said payload (P) in said information signal (X).
 2. A method according to claim 1 wherein the payload (P) is created as a signature dependent, invertible function of said message (M).
 3. A method according to claim 1 wherein the payload (P) is created by concatenating the message (M) and a function of at least one of said signatures (S).
 4. A method according to claim 1, wherein the payload is dependent of at least one random signature from said number of robust signatures (S).
 5. A method according to claim 1, wherein the payload is dependent of at least the n-th signature from said number of robust signatures (S).
 6. A method according to claim 1, wherein the payload is dependent of at least the most robust signature from said number of robust signatures (S).
 7. A method according to claim 1, wherein the payload is dependent of signature chosen from said number of robust signatures (S), the signature being chosen using a key-frame.
 8. A method of detecting a message (M) in a time dependent information signal (Y) with an embedded watermark pattern (W), said method comprising the steps of: detecting the watermark pattern (W) in said information signal (Y) and detecting the payload (P), determining a number of robust signatures, each signature (S) being a set of robust features in said information signal (Y), said robust signatures being determined in a predetermined time interval of said information signal (Y), decoding said message (M) from said payload (P) using at least one of said number of robust signatures.
 9. A method according to claim 8 wherein the message (M) is decoded from the payload (P) using a signature dependent, invertible function of said payload (P).
 10. A method according to claim 8 wherein payload (P) has been created by concatenating the message (M) and a function of at least one of said signatures (S) and wherein the step of decoding the payload (P) comprises means for selecting the relevant part of said payload (P)
 11. A method according to claim 10 wherein the decoding of said message from said payload comprises the step of checking that the remaining part of the payload is a function of at least one of said signatures (S).
 12. A method according to claim 8, wherein the decoding of said message (M) from said payload (P) is done using the n-th signature from said number of robust signatures (S).
 13. A method according to claim 8, wherein the decoding of said message (M) from said payload (P) is done using the most robust signature from said number of robust signatures (S).
 14. A method according to claim 8, wherein the decoding of said message (M) from said payload (P) is done by consecutively using all the signatures from said number of robust signatures (S).
 15. An arrangement for embedding a watermark pattern (W) with a payload (P) in a time dependent information signal (X), comprising: means for determining a number of robust signatures (S), each signature (S) being a set of robust features in said information signal (X), said robust signatures being determined in a predetermined time interval of said information signal, means for creating the payload (P) being dependent of at least one of said number of robust signatures (S) and a predefined message (M), means for embedding said watermark pattern (W) according to said payload (P) in said information signal (X).
 16. An arrangement for detecting a message (M) in a time dependent information signal (Y) with an embedded watermark pattern (W), comprising: means for detecting the watermark pattern (W) in said information signal (Y) and detecting the payload (P), means for determining a number of robust signatures, each signature (S) being a set of robust features in said information signal (Y), said robust signatures being determined in a predetermined time interval of said information signal (Y), means for decoding said message (M) from said payload (P) using at least one of said number of robust signatures.
 17. A device for recording and/or playing back an information signal, the device comprising means (402) for disabling recording and/or playback of the information signal in dependence upon a watermark pattern (W) embedded in said information signal wherein the device further comprises an arrangement (404) for detecting said basic watermark pattern (V) according to claim
 16. 18. A device for transmitting an information signal, the device comprising an arrangement for embedding a watermark pattern (W) with a payload (P) in a time dependent information signal (X), comprising: means for determining a number of robust signatures (S), each signature (S) being a set of robust features in said information signal (X), said robust signatures being determined in a predetermined time interval of said information signal, means for creating the payload (P) being dependent of at least one of said number of robust signatures (S) and a predefined message (M) means for embedding said watermark pattern (W) according to said payload (P) in said information signal (X).
 19. An information signal (Y) with an embedded watermark pattern (W) and a payload (P), said payload (P) being dependent of at least one of a number of robust signatures in said information signal (Y) and a predefined message (M).
 20. An information signal (Q) according to claim 19, wherein the payload (P) is a signature dependent, invertible function of said message (M).
 21. A storage medium (401) having stored thereon an information signal (Y) with an embedded watermark pattern (W) and a payload (P), said payload (P) being dependent of at least one of a number of robust signatures in said information signal (Y) and a predefined message (M). 